This invention relates to apparatus and methods of generating tones and announcements for use with telephone networks and, more particularly, to apparatus and methods of doing so by generating both such tones and announcements in only digital form and on a time division multiplex, multichannel basis.
It is known to store binarily encoded tones and selectively provide them to output ports on a multichannel, time division multiplex basis to a telephone switching network, as shown in U.S. Pat. No. 4,205,203 issued May 27, 1980, to Mehta et al. and owned by the assignee of this application. In such case, different ones of a plurality of tones are digitally signaled selectively in the different time slots of recurring frame cycles. Generally, a group of binarily encoded, or PCM, samples for each tone are stored in addressable memory. As each given time slot recurs, one sample of a tone preselected for that time slot is read out to a common bus As successive cycles occur, the address signals applied to the memory during any given time slot are progressively changed to cause repeated scanning of the samples of the preselected tone. The preselection permits any of the sounds to be placed in any of the time slots, or channels. Such binarily encoded tones are stored in nonvolatile static, read only memories or ROMS. While no means are discussed for storing announcements, the ROM's used to store the codes could also be used to store announcements if the storage capacity were sufficient.
An announcer known as the Cook Electric Model 213300 Single Channel Digital Announcer manufactured by Northern Telecon Inc. is also known which uses only RAM to store only binarily encoded announcements. The announcer is shown and described in the Operators and Maintenance Manual, published 1982, by Northern Telecon Inc., Cook Electric Division. While tones could also be stored in the RAM, the announcer of Cook Electric Division has no means for generating tones; only announcements are generated. There are no ROM's for tone storage; only RAM's are available. In order to protect against loss of binarily encoded announcements stored in RAM, in the Cook Electrical Company announcer, a back-up circuit provides ancillary battery power to the RAM in the event of loss of regular power.
RAM memory is relatively less expensive than ROM memory. The disadvantage of using RAM memory is that it is a volatile memory meaning that storage of information will only last as long as electrical power is applied thereto. In the event of power loss, any binarily encoded tones stored in RAM would be lost and the tone memories must be reloaded off line. Fortunately, in order to store a tone which consists of a repeating pattern, only the smallest segment containing the entire pattern needs to be stored. Accordingly, nonvolatile but more expensive ROM's can be used in place of the less expensive RAM's since only a relatively small amount of memory is required. Announcements, on the other hand, do not comprise rapidly repeating patterns and require much more storage than required for tones.
The problem with the tone generator of the aformentioned Mehta et al. patent, is that only tones can be generated and only expensive ROM's are provided for storage of tones or announcements. While using only ROM's eliminates the need for a back-up circuit to provide ancillary power in the event of power loss, the cost of the ROM's for announcement storage is excessive. Conversely, the disadvantage of the aforementioned Cook Electric Division announcer is that only RAM's are provided for storage of both announcements and tones. Accordingly, even though a back-up battery is provided, should the back-up circuit fail, not only will announcements be lost but also any tones would be lost if stored in RAM in the event of regular power loss.
Another problem with the aforementioned Cook Electric Division announcer is that it only contains a single channel for announcements and interfaces with the telephone network through an analog interface containing expensive A/D converters even though the announcements are stored in digital binarily encoded form. These A/D converters also cause increased distortion, drift and generally degrade the signals which have been stored digitally.